Much of the lifeblood of the world economy flows through pipeline transportation systems. Large volumes of products as diverse as petroleum and liquid hydrocarbons, natural gas, propane, and slurries of solids such as granulated coal and minerals such as copper and iron are constantly being transported between production sites and processing and consumption sites over long distances. These pipelines range generally between 12 inches and 60 inches in diameter and extend to thousands of miles in length. In addition, there are curves and bends along the pipeline with radii of curvature of generally about three times the pipeline diameter, though tighter bends are possible. Usually constructed of metal, in particular, ferrous metals, pipelines are susceptible to damage and other defects which affect the integrity of the system. The result can be a failure which threatens life and property, serious environmental damage, disruptions to both local and distant economies, and loss of the product being transported. The further result can be reduced public confidence in this efficient and economic means of transporting materials with possible public opposition to the growth of such means.
Pipelines, such as water mains and sewers, are vital to the quality of life of individual citizens and to the economic productivity of society. Over time, water pipelines will deteriorate, and eventually, they will fail entirely. Keeping these lines operable is a challenge faced by every community, both in terms of maintenance and repair costs and in terms of engineered capacity. In meeting these challenges, it is essential to have accurate information on the condition of the pipeline. Traditionally, communities have relied on indirect methods of deterioration detection, e.g., visible leaks, soil corrosion potential, statistical pipe break frequencies, pressure drops, soil settlement, etc., or by manually exhuming a portion of the pipeline in order to extrapolate the condition of the entire pipeline.
Many water distribution systems throughout the world have been in use for periods approaching or exceeding a century. Over time, the water systems have received varying degrees of maintenance, however, inspection is difficult without costly excavation. Often, no action is taken until a leak is detected, at which time the section surrounding the leak is excavated and repaired. System maintenance has often been limited to monitoring the failure rates for individual lines and performing replacement of an entire line or a long segment of it when leak frequency has exceeded tolerable values. This approach may lead to unnecessary replacement of considerable good pipe. As a result, there exists a need for a cost effective method to ascertain line condition. Since water lines are almost always buried, any applicable inspection method must be capable of operating solely within the bore of the pipe, to detect flaws such as corrosion and cracks through the entire thickness of the pipe.
In order to make inspection cost effective, it must be possible to perform the inspection with minimal preparation of the line, and, in particular, without having to excavate the lines. This means that the inspection device must be capable of accessing the line through existing access points, such as hydrants. The water pressure in lines is generally about 80 PSIG, and can reach pressures of 120 PSIG. The inspection device must be able to withstand such water pressures.
The inspection method must be useable with pipes made of inhomogeneous materials, such as cast iron. In addition, the presence of right-angle elbows and tees, large numbers of service taps and fittings, and the relatively large accumulations of scale typical of municipal water systems requires the use of a device which is flexible and able to flex around bends and fit through small openings.
There are several methods of inspection which offer the possibility of measuring pipe condition from the inside, and which are used for this purpose in other applications. Among these are ultrasonic, magnetic flux leakage, eddy current, and remote field eddy current technology.
Ultrasonic methods are used extensively to measure the thickness of many materials with one sided access only, and exhibit very good accuracy in most steels. Unfortunately, they do not work well in cast iron, because the grain size in cast iron approaches the ultrasonic wave length. This results in severe scattering and attenuation of the acoustic signal.
Flux leakage methods are used extensively in oil well casing and petroleum pipeline applications. They are limited by the requirements that the pipe be very clean inside to prevent sensor bounce, and that a substantially constant speed be maintained. The scale build-up typical of water lines prevents flux leakage inspection, as does the relatively great wall thickness of these lines. In addition, while this method is effective for the detection of localized sharp edged pits and cracks, it is very insensitive to general overall wall loss.
Eddy current methods have been the technique of choice for many years in the inspection of non-magnetic metal piping in applications such as air conditioning units and non-ferrous chemical process piping. In magnetic materials such as cast irons and carbon steels, the depth of penetration of eddy currents is greatly reduced, precluding inspection of the outside of the pipe, particularly when the pipe is of appreciable thickness. Attempts have been made to overcome this limitation by the use of constant magnetic fields to reduce the effective magnetic permeability of the material, but the thickness of typical water lines and the presence of scale make this method impractical for the inspection of these lines. Also, eddy current probes react strongly to changes in the distance between the sensors and the material under inspection, which requires that the inside of the pipe be very clean. For these reasons, this is not a viable method for water line applications.
Remote field eddy current (RFEC) is a relatively new electromagnetic inspection method which has become prominent in the last few years. The term “remote field eddy current” is used to describe the technique in which an alternating magnetic field is induced in the pipe by an excitation or source coil and the field as modified by the pipe material is detected at a location remote from the exciter coil. The detector must be spaced from the exciter coil a sufficient distance to eliminate direct coupling within the pipe between the exciter coil and the detector, and thereby overcome the problems associated with traditional eddy current methods. From classic eddy current equations one can derive an equation illustrating that flux density at any depth will be attenuated and delayed in time (shifted in phase) in a manner related to metal thickness. In particular, eddy current instruments detect a flaw by measuring the reduced attenuation, time delay and field direction the flaw produces as compared with a normal wall thickness. This perturbation in the inner wall electromagnetic field pattern caused by a flaw is highly localized in the vicinity of the flaw and will, to a limited extent, outline the shape of the flaw.
Utilities are responsible for the regular maintenance and integrity of their underground infrastructure. To minimize the risk of failure, pipelines are closely monitored and inspected. However, the methods described above are generally costly and difficult to apply.
There is a need for a means and a method for inspecting a buried pipeline or a utility which is easy and efficient.